1. Field of the Invention
Our invention generally relates to means for roll-compensation in a mobile electromagnetic antenna array and, more specifically, to means for minimizing the roll polarization loss in an array of unipolar elements.
2. Description of the Related Art
The use of planar array seeker antennas on mobile platforms such as watercraft and aircraft is well-known in the art but introduces several difficult problems. One of these problems is the requirement to compensate for polarization loss in the antenna signal resulting from platform maneuvering about an axis parallel to the planar array normal. This requirement arises because of the linear polarization of the unipolar elements making up the planar array. Each array element provides an electrical output signal in response to an incident electromagnetic plane wave polarization component aligned with the polarization orientation of the unipolar antenna element in a manner well-known in the art. See, for instance, Warren B. Offutt, et al., "Chapter 23: Methods of Polarization Synthesis", Antenna Engineering Handbook, 2nd ed., Richard C. Johnson, et al., Eds., McGraw-Hill Book Company, 1984.
The use of a planar array configuration provides the advantage of increased sensitivity because of the directivity gain of such an array and the advantage of electronic steerability of the antenna beam given the appropriate beam steering electronics. The increased directivity makes such an antenna sensitive to platform maneuvering in yaw and pitch but automatic electronic beam steering techniques known in the art can easily compensate for the degradation of antenna sensitivity with yaw and pitch maneuvers. The polarization loss caused by roll maneuvering is not so easily overcome because it results from reduced sensitivity of the individual array elements to misaligned incoming signal polarization angles. Practitioners in the art have attempted to solve this problem by reducing element sensitivity to polarization alignment and also by physically holding the planar array stable in space during roll maneuvers of the aircraft.
The polarization of an electromagnetic wave is defined by the direction in which the electric vector is aligned during the passage of at least one full cycle. Generally, both the magnitude and direction of the electric vector will vary during each cycle and the electric vector will map out an ellipse in the plane normal to the direction of propagation. The direction of the major elliptical axis is the polarization orientation, which is normally defined as an angle .beta. from an arbitrary vertical. If the major and minor axes of the ellipse are equal, the wave is said to be circularly polarized. Also, if the minor axis is substantially zero in magnitude, the wave is said to be linearly polarized. Thus, a linearly polarized wave is defined as a transverse electromagnetic wave whose electric field vector at all times during the cycle lies along a fixed line at some tilt angle .beta. with respect to the vertical.
Any wave of arbitrary polarization can be synthesized from two unipolar waves of orthogonal polarization. A circularly polarized wave results from the combination of a vertically and a horizontally polarized wave of identical amplitude having a ninety (90.degree.) degrees phase difference between them. The same waves having equal amplitude but no phase difference will combine to form a linearly polarized wave with a 45.degree. orientation with respect to the vertical. Thus, as should be well-known in the art but is often confused, the use of a circularly polarized antenna element to detect a linearly polarized wave of arbitrary orientation will not alone avoid losses associated with changes in the phase relationship between the horizontal and vertical components of such an arbitrary wave. This problem limits the effectiveness of simple circularly polarized elements for planar array roll stabilization techniques involving isolation of polarization components, primarily because it is not suitable for planar arrays requiring off-axis beam-steering.
Nevertheless, previous solutions proposed for the problem of increasing polarization loss with roll angle for planar arrays of linearly polarized elements often rely on substitution of circularly polarized elements. A simple and well-known method for converting linearly polarized elements into a circularly polarized element is to orthogonally combine two such elements in quadrature by phase shifting one element output signal 90.degree. before adding the second element output signal form a second orthogonally-disposed element. Such quadrature schemes require electronic circuits capable of adjusting the signal time delay as a function of carrier frequency, which is known to be complex and difficult as well as expensive. Alternatively, two such orthogonal elements can be displaced by one-quarter wavelength along an axis in the direction of propagation, thereby achieving the necessary 90.degree. quadrature phase shift between elements, but such a physical spacing is accurate at only a single frequency.
It is also well-known that circular polarization can be achieved by a combination of dissimilar electromagnetic antennas if the fields produced are equal in magnitude and in time-phase quadrature. A simple example of such a combination is the horizontal loop and a vertical monopole. In practice, this combination is useful only over narrow bandwidths because of dissimilar impedance characteristics.
Another circularly polarized combination known in the art consists of two vertical half-wavelength cylinders in which vertical slots are cut. The two cylinders provide a vertically polarized omnidirectional pattern and the two slots give a horizontally polarized pattern in the same plane. If the two radiated signals are carefully adjusted so that they are in time-phase quadrature, the resulting pattern will be circularly polarized.
Several other circular polarization techniques are known in the art, but most provide circular polarization only on axis and all such techniques tend to be limited in bandwidth because of the precise phase relationships required between the combined elements.
The second scheme known in the art for minimizing polarization losses during roll maneuvers includes methods for physical array antenna stabilization during roll maneuvers. These methods vary in complexity and effectiveness but generally involve inertial sensing means in combination with rotational motor means for rotating the planar array antenna about the aircraft roll axis to stabilize the physical antenna orientation with respect to a stable reference frame. Obviously, the cost, complexity and reliability of these schemes makes them generally less desirable than other non-mechanical solutions to the roll-compensation problem.
The simplest and least expensive solution to the general problem of polarization loss would ideally involve an array of linearly polarized elements that requires no special phase shift circuitry and no quadrature summation means. For instance, U.S. Pat. No. 3,283,330 issued to Maurice G. Chathelain on Nov. 1, 1966, discloses an omnipolarization microstrip antenna that is simple and economical to manufacture using minimal components. Chathelain attains this simplicity by using linearly polarized elements arranged along a microstrip to provide omnipolarization characteristics to an endfire array pattern. Chathelain teaches the use of an array of monopoles extending from a ground plane and inclining outwardly from said microstrip. The spacing of his inclined monopoles is staggered on either side of the microstrip as necessary to provide the phase relationship required for circular polarization of radiation propagated at endfire. However, Chathelain's technique is not applicable for planar arrays having broadside or beamsteered radiation patterns. Moreover, Chathelain's technique is limited to monopole elements and is not practical for application to arrays using slot radiator elements or other linearly polarized elements known in the art.
The use of unipolar elements disposed with a tilted polarization orientation with respect to the vertical array axis has been suggested in the prior art for a variety of purposes but all such planar array techniques teach the use of identical element tilts throughout the entire array. Such an identical tilt scheme does nothing to control polarization loss with respect to roll angle because, as mentioned above, merely tilting a series of linearly polarized elements is nothing more than a change to the effective direction of the arbitrary vertical reference and has no effect on antenna sensitivity to misaligned polarization components. Thus, there is a strongly felt need in the art for a simple, inexpensive and accurate means for overcoming the polarization loss associated with roll maneuvers with planar array aircraft antennas. These unresolved problems and deficiencies are clearly felt in the art and are solved by our invention in the manner described below.